TN-LCD (twisted nematic liquid crystal display device) is a typical liquid crystal display device and used in clocks, electronic calculators, electronic pocketbooks, pocket computers, word processors, personal computers and the like. On the other hand, with the increase in the information contents of OA instruments, STN (super twisted nematic)-LCD has been developed by Scheffer et al. (SID '85 Digest, p. 120, 1985) and Kinugawa et al. (SID '86 Digest, p. 122, 1986) and is now popularizing in the field of portable terminals, electronic pocketbooks, pocket computers, word processors, personal computers, monitor displays and the like high information displays.
Recently, an active addressing drive system (Proc. 12th IDRC, p. 503, 1992) and multiline addressing drive system (SID '92 Digest, p. 232, 1992) have been proposed for the purpose of improving response characteristics of STN-LCD. Also, with the aim of achieving brighter display or higher contrast ratio, a novel reflection type liquid crystal color display system in which birefringence of liquid crystal and retardation film was used in stead of color filter layers (The Institute of Television Engineers of Japan, Technical Reports, vol. 14, No. 10, p. 51, 1990) and a liquid crystal display device having a refracting surface in which a small parboloid is arranged on the substrate electrode side have been proposed.
Particularly, uniform and high contrast display against the temperature distribution of back light is expected for the purpose of enlarging the display area, so that liquid crystal materials having more stable orientation and smaller temperature-dependency are expected, and a birefringence corresponding to a predetermined value is also expected for reducing dispersion of cell thickness. Since high duty drive is carried out by increasing the number of pixels, response, gradient and the like are also regarded as important factors. In the case of medium and small size portable displays, on the other hand, stability of display against working environmental temperature is an important point, so that liquid crystal materials having lower driving voltage which can reduce response and consumptive electric power are expected, as well as smaller temperature dependency of driving voltage, sharpness and frequency-dependency of desired duty drive within the temperature range of from -30 to 0.degree. C. or from 40 to 80.degree. C. In addition, though it is necessary to avoid too low electrical resistance (specific resistance) for saving consumptive electric power, it is required to set the resistance to a predetermined value so that it does not become too high which will cause image sticking. Thus, liquid crystal materials which are differentiated further in detail and improved even to a certain degree are still in demand.
As has been described in detail in the foregoing, there are many demands for liquid crystal display devices, such as more precise and high density display capacity, quicker response speed (switching time) against driving voltage and environmental temperature, lower driving voltage having chemically and electrically high stability, higher gradient and higher contrast for working environmental temperature and viewing angle. Because of this, research and development are still carried out on liquid crystal materials which can achieve desired driving voltage, particularly lower driving voltage, with lower viscosity so that the response property can be improved while having the nematic property within a wide range of temperature and maintaining the nematic phase for a prolonged period of time under low temperature storage conditions. In addition, designing of birefringence, dielectric anisotropy and elastic constant and their temperature-dependency, light wave length-dependency of birefringence and frequency-dependency of dielectric anisotropy in response to duty numbers are also drawing attention as means to be improved.
To meet such demands for liquid crystal materials, it is necessary to generally optimize birefringence, elastic constant, dielectric anisotropy, lower viscosity, wider nematic temperature range, chemical stability, electric stability (desired specific resistance) and the like physical characteristics and predetermined tilt angle concerning orientation property, wider d/p margin and the like individual characteristics, and the development of new liquid crystal compounds or liquid crystal compositions is still in demand.
With regard to the compounds related to the groups A1 to A3 of the present invention, which will be described in the following, compounds of the general formula (I-1) are described for example in German Patent 2306739 (1973) and J. Phys. (Paris), Suppl. 36, C1, 387 (1975), compounds of the general formula (I-4) are described in JP-A-58-83665 (1983) (the term "JP-A" as used herein means an "unexamined published Japanese patent application") and U.S. Pat. No. 4,455,261 (1984) and compounds of the general formulae (I-2), (I-3) and (I-5) to (I-9) are described in JP-W-3-503637 (the term "JP-W" as used herein means an "unexamined published Japanese international patent application") and WO 89/08102 (1989). Compositions in which these compounds are used are described for example in JP-A-9-157654 (1997) in addition to the just described references.
With regard to compounds related to the groups A4 to A6 of the present invention, which will be described in the following, compounds of the general formulae (I-10) and (I-13) are described in JP-A-2-225444 (1990), JP-A-2-233656 (1990), European Patent 464648 (1991), European Patent 466183 (1991) and JP-A-4-230352 (1992), and compounds of the general formula (I-16) are described for example in JP-A-4-300861 (1992).
In addition to the aforementioned references, compositions in which these compounds are used are described for example in JP-W-62-501509 (1986), U.S. Pat. No. 4,818,426 (1989), European Patent 207975 B (1990), JP-W-63-502596 (1987), JP-W-1-500837 (1988), JP-A-4-279560 (1992), WO 96/32365 (1996), JP-A-9-59625 (1997), JP-A-9-227866 (1997), JP-A-10-88140 (1998), JP-A-10-140157 (1998) and JP-A-10-158651 (1998).
However, the aforementioned techniques, even some of them are unrealized, are not sufficient for achieving objects of the present invention. For example, compounds of the general formulae (I-11) and (I-12) and compounds of the general formulae (I-14) and (I-15) or the general formulae (I-17) and (I-18) of the present invention are not known. With regard to techniques concerning the compositions, descriptions about general combinations of compounds can be found illustratively, but their illustrative examples are rare, so that technical disclosures which can be used easily by those skilled in the art are not sufficient yet.
In addition, even if the aforementioned techniques are used, problems still remain unsettled. For example, great concern has been directed toward the improvement of the response property of liquid crystal display by smaller viscoelasticity in comparison with the size of dielectric anisotropy .DELTA..epsilon. or the realization of liquid crystal display which can be driven within a wide range of temperature by obtaining a nematic phase that is stable at a low temperature through the reduction of a crystallizing or precipitating tendency against other liquid crystal materials. Also, in the case of word processors, personal computers and the like STN-LCDs having large information capacity, small frequency dependency of driving voltage to cope with high duty, or small temperature dependency of driving voltage in the case of portable use, is expected. Because of this, more excellent new liquid crystal materials are expected.